U.S. patent number 5,425,632 [Application Number 08/088,614] was granted by the patent office on 1995-06-20 for process for burning combustible mixtures.
This patent grant is currently assigned to Catalytica, Inc., Tanaka Kikinzoku Kogyo K.K.. Invention is credited to Ralph A. Dalla Betta, Nobuyasu Ezawa, Kazunori Tsurumi.
United States Patent |
5,425,632 |
Tsurumi , et al. |
* June 20, 1995 |
Process for burning combustible mixtures
Abstract
This invention is a process for catalytically burning a
combustible mixture of a fuel and an oxygen-containing gas. In
particular, the invention is a process for producing a combustion
gas at a selected temperature, preferably between 1050.degree. C.
and 1700.degree. C., by introducing all of the fuel necessary to
attain that temperature to a combustion catalyst, partially
combusting the combustible within the combustion catalyst, and
homogeneously combusting the remainder of the fuel outside the
catalyst. By controlling the temperature within the catalyst,
deterioration of that catalyst is prevented.
Inventors: |
Tsurumi; Kazunori (Fujisawa,
JP), Ezawa; Nobuyasu (Koto, JP), Dalla
Betta; Ralph A. (Mountain View, CA) |
Assignee: |
Catalytica, Inc. (Mountain
View, CA)
Tanaka Kikinzoku Kogyo K.K. (JP)
|
[*] Notice: |
The portion of the term of this patent
subsequent to January 25, 2011 has been disclaimed. |
Family
ID: |
24475830 |
Appl.
No.: |
08/088,614 |
Filed: |
July 6, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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617976 |
Nov 26, 1990 |
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Current U.S.
Class: |
431/7;
502/339 |
Current CPC
Class: |
F23C
6/045 (20130101); F23C 13/00 (20130101); F23C
2900/13002 (20130101) |
Current International
Class: |
F23C
6/00 (20060101); F23C 6/04 (20060101); F23C
13/00 (20060101); F23D 021/00 () |
Field of
Search: |
;431/7,328,170 ;60/723
;502/262,233,339,527 ;422/170,171 ;48/127.7 |
References Cited
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|
Primary Examiner: Price; Carl D.
Attorney, Agent or Firm: Morrison & Foerster
Parent Case Text
This application is a continuation, of application Ser. No.
07/617,976, and filed Nov. 26, 1990 now abandoned.
Claims
We claim as our invention:
1. A catalytic process for producing a combustion gas having a
temperature between 1050.degree. C. and 1700.degree. C., the
process comprising the steps of:
a. partially combusting a combustible mixture comprised of fuel and
oxygen-containing gas and having a theoretical adiabatic combustion
gas temperature in a catalytic structure having at least one stage
wherein a catalyst is situated so that a portion of the combustible
mixture is inhibited in its contact with the catalyst by the
presence of a diffusion layer on the catalyst so that only a
portion of the fuel is combusted within the catalytic structure and
the temperature of the partially combusted combustible mixture
discharged from the catalytic structure is below the theoretical
adiabatic combustion temperature, and
b. combusting the remainder of the fuel in the combustible mixture
outside of the catalytic structure to produce a combustion gas
having a temperature within the desired range of 1050.degree. C.
and 1700.degree. C.
2. The process of claim 1 where the first stage catalyst comprises
palladium, the second stage catalyst comprises palladium, and the
third stage catalyst comprises platinum.
3. The process of claim 2 where the temperature of the combustible
mixture discharged from the first stage is between 500.degree. C.
and 650.degree. C. and that discharged from the second stage is
between 750.degree. C. and 800.degree. C.
4. The process of claim 1 where the catalyst support is a
corrugated metal structure.
5. The process of claim 1 where the catalytic structure in which
catalytic material is situated so that a portion of the combustible
mixture is inhibited in its contact with the catalytic comprises a
catalyst support having catalyst with a diffusion barrier situated
therein.
Description
FIELD OF THE INVENTION
This invention is a process for catalytically burning a combustible
mixture of a fuel and an oxygen-containing gas. In particular, the
invention is a process for producing a combustion gas at a selected
temperature, preferably between 1050.degree. C. and 1700.degree.
C., by introducing all of the fuel necessary to attain that
temperature to a combustion catalyst, partially combusting the
combustible within the combustion catalyst, and homogeneously
combusting the remainder of the fuel outside the catalyst. By
controlling the temperature within the catalyst, deterioration of
that catalyst is prevented.
BACKGROUND OF THE INVENTION
One widely used process for the generation of electricity entails
the use of a fuel-fired turbine to turn a generator. The turbine
turns by the introducing a hot exhaust gas through the turbine. In
this process, catalysts have been used for igniting and burning the
combustible fuel. The fuel and an oxygen-containing gas, typically
air, are mixed and introduced into a combustion apparatus
containing the catalysts. The mixture is burned over the catalysts
and the resulting high temperature exhaust gas is introduced into
the turbine. The efficiency of the generation process is largely
dependent upon the temperature of the gas introduced into the
turbine. That is to say, the higher the temperature of the burned
gas, the higher the efficiency of the turbine at least so long as
the turbine's materials are able to withstand the high
temperatures. A typically appropriate temperature range for modern
gas turbines is between 1300.degree. C. and 1500.degree. C.
Although it is desirable to introduce all of the needed fuel and
oxygen-containing gas needed to reach a desired exhaust gas
temperature into the catalyst, it is quite difficult to control the
temperature within that catalyst.
At present, we do not know of any catalyst which is capable of
operating at the desired turbine gas temperature of 1300.degree. C.
or above for an appreciable period of time without substantial
deterioration of the catalyst. Others have suggested that
controlling catalyst temperatures at a level at which catalyst
deterioration is minimized may be accomplished by introducing the
needed fuel into the catalyst in a series of stages rather than
introducing all of the fuel together. This approach obviously
requires the separation of the catalyst bed into a series of
separate beds in which the temperature rise in each is separately
controlled.
However, even this suggested process does not possess the ability
consistently to produce an exhaust gas at a temperature over 1300
.degree. C. since the catalyst in the latter stages must face that
temperature and consequently will deteriorate. Additionally, since
the fuel is introduced into the catalyst at a number of points, the
apparatus surrounding the catalyst is complex and its operation is
complicated. Exhaust gases containing up to 10 ppm NO.sub.x or more
may be produced because the fuel in the final stage is likely
imperfectly or nonuniformly mixed with the partially combusted
gases from the earlier stages.
In contrast, however, the present invention does not cause the
temperature of the exhaust gas in the catalyst structure does not
rise to a level which will cause the catalyst to undergo
deterioration. The gas will be at a temperature, however, that will
allow homogeneous combustion of uncombusted fuel to occur after the
partially combusted gas leaves the catalyst bed. In other words,
the resulting gas is ultimately produced at a temperature which is
at or above the deterioration temperature of the catalyst without
deteriorating the catalyst.
Additional fuel and air need not be supplied to the intermediate
stages of the of the catalyst since all of the fuel needed to
produce the desired exhaust temperature is supplied initially to
the catalyst. No fuel concentration gradient in the catalyst bed is
needed to suppress NO.sub.x production. Finally the complicated
apparatus needed to supply fuel to the various catalyst stages of
that known process is not needed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic and view of a corrugated catalyst structure
having catalytic material on one side of the structure surface.
FIG. 2 is a schematic representation of the three stage catalytic
reaction apparatus used in the Examples.
FIGS. 3 and 4 are graphs of various operating temperatures as a
function of preheat temperature.
FIG. 5 is a graph of various operating temperatures during a long
term steady state operation test.
FIG. 6 is a graph of various operating temperatures during a
typical start up procedure.
DESCRIPTION OF THE INVENTION
This inventive process avoids the deterioration of the catalysts
employed in the catalytic combustion apparatus by limiting the
temperature in the catalyst to a level less than about, for
example, 1000.degree. C. to 1200.degree. C., or such temperature
that the catalyst does not deteriorate. The gas, as it leaves the
catalyst, will contain some amount of unburned fuel which will be
at a temperature which will undergo homogeneous combustion to a
higher temperature, for example, 1300.degree. C. to 1500.degree.
C., suitable for introduction to a gas turbine. This homogeneous
combustion takes place at a position which is sufficiently remote
from the catalyst so that the catalyst is not harmed. The process
of the present invention contemplates initially supplying to the
catalyst, all fuel needed for the ultimately desired
temperature.
This process may be used with a variety of fuels and at a broad
range of process conditions.
Although normally gaseous hydrocarbons, e.g., methane, ethane, and
propane, are highly desirable as a source of fuel for the process,
most fuels capable of being vaporized at the process temperatures
discussed below are suitable. For instance, the fuels may be liquid
or gaseous at room temperature and pressure. Examples include the
low molecular weight hydrocarbons mentioned above as well as
butane, pentane, hexane, heptane, octane, gasoline, aromatic
hydrocarbons such as benzene, toluene, ethylbenzene; and xylene;
naphthas; diesel fuel, kerosene; jet fuels; other middle
distillates; heavy distillate fuels (preferably hydrotreated to
remove nitrogenous and sulfurous compounds); oxygen-containing
fuels such as alcohols including methanol, ethanol, isopropanol,
butanol, or the like; ethers such as diethylether, ethyl phenyl
ether, MTBE, etc. Low-BTU gases such as town gas or syngas may also
be used as fuels.
The combustion catalysts employed in this invention may be of a
single type on an appropriate support but, because the catalyst
structure may desirably be separated into a number of stages,
different catalysts may be used in different stages.
The preferred supports for the catalyst of this invention comprise
metal, inorganic oxides, or ceramics. Suitable ceramic support
materials are known in the art. Various appropriate inorganic
oxides which may be used as supports include silica, alumina,
silica-alumina, titania, zirconia, etc., and may be used with or
without additives such as barium, cerium, lanthanum, or chromium
added for stability. Metallic supports in the form of honeycombs,
spiral rolls of corrugated sheet (which may be interspersed with
flat separator sheets), columnar (or "handful of straws"), or other
configurations having longitudinal channels or passageways
permitting high space velocities with a minimal pressure drop are
desirable in this service.
One way contemplated by the inventors for limiting the temperature
of the catalyst to an acceptable value involves placement of the
catalyst in the support so that only a portion of the combustible
gas is in contact with a catalyst as it passes through the catalyst
structure and the remaining portion of the gas is merely in contact
with the support. This segregation of gas is accomplished by
placing catalyst on only a number of the longitudinal passageways
through the catalyst support while maintaining other passageways
catalyst-free. In this way, the fuel in the combustible mixture
flows through the catalyst-free passageways without being burned.
It is additionally desirable to place the catalysts in the
passageways so that the catalytic materials are in heat exchange
relationship with the catalyst-free passageways.
The process of this invention desirably employs a three stage
reaction apparatus having palladium as the catalytic material in
the first two stages and using platinum as the catalytic material
in the third stage. The corrugated supports mentioned above may be
used in any stage but desirably are used at least in the second and
third stages. Other catalytic materials may be utilized in the
third stage in place of palladium and platinum, including, such
materials as the other platinum group metals, base metal (Fe, Mn,
Co, etc.) oxides, and refractory metal oxides.
Another method for inhibiting the combustible mixture's contact
with the combustion catalyst involves forming a barrier layer on
the catalyst. Suitable barrier materials include alumina, silica,
zirconia, titania, and other inorganic oxides having low catalytic
combustion activity. Alumina is the least desirable of these
materials.
The process normally would be practiced at the operating pressure
of the gas turbine. Compression of combustion air to the operating
pressure typically would produce an air stream at a temperature of
about 300.degree. C. This stream is then mixed with the fuel stream
and introduced into the first stage catalyst. The fuel in the
compressed combustible mixture then ignites and the temperature of
the mixture rises. The partially combusted stream then passes to
the second catalytic stage where the temperature continues to rise
because of its contact with the palladium catalyst contained in
that stage. As was noted above, it is desirable to use a catalyst
support having catalyst on only a portion of the catalyst support
passageways in this stage. Only a portion of the uncombusted fuel
is therefore burned in this stage and the temperature rise is
moderated.
An additional reason for the moderation of the temperature found in
these earlier stages lies in the use of the palladium catalyst.
Palladium is very active at 325.degree. C. and lower for methane
oxidation and can "light off" or ignite fuels at low temperatures.
It has also been observed that in certain instances, after
palladium initiates the combustion reaction, the catalyst rises
rapidly to temperatures of 750.degree. C. to 800.degree. C. at one
atm of air or about 940.degree. C. at ten arm total pressure of
air. These temperatures are the respective temperatures of the
transition points in the thermogravimetric analysis (TGA) of the
palladium/palladium oxide reaction shown below at the various noted
pressures. At that point the catalytic reaction slows substantially
and the catalyst temperature moderates at 750.degree. C. to
800.degree. C. or 940.degree. C., depending on pressure. This
phenomenon is observed even when the fuel/air ratio could produce
theoretical adiabatic combustion temperatures above 900.degree. C.
or as high as 1700.degree. C.
One explanation for this temperature limiting phenomenon is the
conversion of palladium oxide to palladium metal at the TGA
transition point discussed above. At temperatures below 750.degree.
C. at one atm of air, palladium is present mainly as palladium
oxide. Palladium oxide appears to be the active catalyst for
oxidation of fuels. Above 750.degree. C. to 800.degree. C.,
palladium oxide converts to palladium metal according to this
equilibrium:
Palladium metal appears to be substantially less active for
hydrocarbon oxidation so that at temperatures above 750.degree. C.
to 800.degree. C. the catalytic activity decreases appreciably.
This transition causes the reaction to be self-limiting: the
combustion process rapidly raises the catalyst temperature to
750.degree. C. to 800.degree. C. where temperature self-regulation
begins. This limiting temperature is dependent on O.sub.2 pressure
and will increase as the O.sub.2 partial pressure increases.
A Mendelev Group IB or IIB metal may be added to the palladium as a
catalyst adjunct. The addition of the adjuncts to the palladium
catalyst shifts the equilibrium or self-limiting temperature of the
resulting catalyst downward. The preferred adjunct metal is silver.
It may be added by incorporating it into the a liquid carrier as a
complex, compound or metal dispersion. After the liquid carrier is
applied to the carrier, it may be decomposed by heat and the
resulting substrate calcined. For instance, silver may be added as
silver acetate, silver nitrate, or an organic silver complex. The
metal is preferably added to make a molar ratio of the adjunct
metal to the palladium in the range of 0.05 to 0.8. A preferred
range is a ratio between 0.3 to 0.3.
It is also possible to control the temperature in the first two
stages by incorporating barrier layers on the catalysts.
In any event, the partially combusted gas is then passed to the
third stage. The desired platinum of the third stage is not
oxidized in the same manner as is palladium. The third stage
desirably utilizes the catalytic and catalyst-free passageways
noted above. Consequently, a portion of the uncombusted fuel
entering the third stage remains uncombusted and thereby moderates
the temperature increase of the third stage so that the resulting
gas temperature reaches a level of about 1000.degree. C. to
1200.degree. C.
Because of the choice of catalyst and catalyst structures and the
fact that the gas leaving the third stage contains uncombusted
fuel, that mixture is at a temperature where the combustion
continues after it leaves the catalyst. No flame occurs however and
the NO.sub.x remains at a low level. In contrast to the prior art
methods where additional fuel is added to the final stage, the
inventive process of this invention does not do so and eliminates
the complexity associated with such an addition.
In the practice of the inventive process, the temperature of the
exhaust gas after the homogeneous combustion is at a level of about
1300.degree. C. to 1500.degree. C. without the addition of more
fuel. This gas temperature approaches the adiabatic combustion
temperature for the particular combustible mixture of fuel and
oxygen-containing gas at the pressure of operation. This gas
temperature level is sufficient so that it may be used effectively
and efficiently in the operation of a gas turbine. Yet the gas
produces no pollution problems in that the level of NO.sub.x
production is practically nil.
Although the present invention has been described in connection
with the operation of a high temperature gas turbine, this
inventive process is not limited to the use of the product gas in
such a turbine.
EXAMPLES
Example 1
A three stage catalyst system was assembled.
Stage 1
The first stage was prepared as follows:
A 3% palladium/ZrO.sub.2 sol was prepared. A sample of 145 g of
ZrO.sub.2 powder with a surface area of 45 m.sup.2 /gm was
impregnated with 45 ml of a palladium solution prepared by
dissolving Pd(HN.sub.3).sub.2 (NO.sub.2).sub.2 in HNO.sub.3
containing 0.83 g palladium/ml. This solid was dried, calcined in
air at 500.degree. C., and loaded into a polymer lined ball mill
with 230 ml H.sub.2 O, 2.0 ml concentrated HNO.sub.3, and
cylindrical zirconia media. The mixture was milled for eight
hours.
To 50 cc of this sol (containing about 35% solids by weight) 36 ml
of palladium solution was added. The pH was adjusted to about nine
and 1.0 ml of hydrazine added. Stirring at room temperature
resulted in the reduction of the palladium. The final palladium
concentration was 20% palladium/ZrO.sub.2 by weight.
A cordierite monolithic ceramic honeycomb structure with 100 square
cells per square inch (SCSI) was immersed in the
palladium/ZrO.sub.2 sol and the excess sol blown from the channels.
The monolith was dried and calcined at 850.degree. C. The monolith
contained 6.1% ZrO.sub.2 and 1.5% palladium. This monolith was
again dipped in the same palladium/ZrO.sub.2 sol but only to a
depth of ten mm, removed, blown out, dried, and calcined. The final
catalyst had 25% ZrO.sub.2 and 6.2% palladium on the inlet ten mm
portion.
Stage 2
The second stage catalyst was prepared as follows:
A ZrO.sub.2 colloidal sol was prepared. About 66 g of zirconium
isopropoxide was hydrolyzed with 75 cc water and then mixed with
100 g of ZrO.sub.2 powder with a surface area of 100 m.sup.2 /gm
and an additional 56 ml of water. This slurry was ball milled in a
polymer lined ball mill using ZrO.sub.2 cylindrical media for eight
hours. This colloidal sol was diluted to a concentration of 15%
ZrO.sub.2 by weight with additional water.
An Fe/Cr/Al alloy foil was corrugated in a herringbone pattern and
then oxidized at 900.degree. C. in air to form alumina whiskers on
the foil surface. The ZrO.sub.2 sol was sprayed on the corrugated
foil. The coated foil was dried and calcined at 850.degree. C. The
final foil contained twelve mg ZrO.sub.2 /cm.sup.2 foil
surface.
Palladium 2-ethylhexanoic acid was dissolved in toluene to a
concentration of 0.1 g palladium/mi. This solution was sprayed onto
one side only of the ZrO.sub.2 coated metal foil and the foil dried
and calcined at 850.degree. C. in air. The final foil contained
about 0.5 mg palladium/cm.sup.2 of foil surface.
The corrugated foil was rolled so that the corrugations did not
mesh to form a final metal structure of two inch diameter and two
inch length with longitudinal channels running axially through the
structure and containing about 150 cells per square inch. The foil
had palladium/ZrO.sub.2 catalyst on one surface only and each
channel consisted of catalytic coated and non-catalytic surfaces
such as those shown in FIG. 1.
Stage 3
The third stage catalyst was prepared as follows:
An alumina sol was prepared. About 125 g of a gamma alumina with a
surface area of 180 m.sup.2 /g, 21 ml of concentrated nitric acid,
and 165 ml of water were placed in a half gallon ball mill with
cylindrical alumina grinding media and milled for 24 hours. This
sol was diluted to a solid concentration of 20%. An Fe/Cr/Al alloy
foil was corrugated to form uniform straight channels in the foil
strip. When rolled together with a flat foil strip, the spiral
structure formed a honeycomb structure with straight channels. The
corrugated strip was first sprayed with a 5% colloidal boehmite sol
and then with the alumina sol prepared above. A flat strip of metal
foil was sprayed in a similar fashion. Only one surface of each
foil was coated in this manner. The foils were then dried and
calcined at 1100.degree. C.
Pt(NH.sub.3).sub.2 (NO.sub.2).sub.2 was dissolved in nitric acid to
produce a solution with 0.13 g platinum/mi. This solution was
sprayed onto the coated foil, the foil treated with gaseous H.sub.2
S, dried, and calcined at 1100.degree. C. The "thickness" of the
alumina coating on the metal foil was about four mg/cm.sup.2 of
flat foil surface. The platinum loading was about 20% of the
alumina.
Three Stage Catalyst System
The three catalysts described above were arranged inside a ceramic
cylinder as shown in FIG. 2. Thermocouples were located in this
system at the positions shown. The thermocouples located in the
catalyst sections were sealed inside a channel with ceramic cement
to measure the temperature of the catalyst substrate. The gas
thermocouples were suspended in the gas stream. The insulated
catalyst section of FIG. 2 was installed in a reactor with a gas
flow path of 50 mm diameter. Air at 1500 SLPM was passed through an
electric heater, a static gas mixer, and through the catalyst
system. Natural gas at 67 SLPM was added just upstream of the
static mixer. The air temperature was slowly increased by
increasing power to the electric heater. At 368.degree. C., the gas
temperatures from stages 1, 2, and 3 began to rise as shown in FIG.
3. Above a preheat temperature of 380.degree. C., the gas
temperature from stage 1 was constant at about 530.degree. C., the
gas exiting stage 2 was about 780.degree. C., and the gas exiting
stage 3 at approximately 1020.degree. C. Homogeneous combustion
occurred after the catalyst giving a gas temperature of about
1250.degree. C.; a temperature near the adiabatic combustion
temperature of this fuel/air ratio. The substrate temperatures for
the three stages are shown in FIG. 4.
As was described above, the stage 1 catalyst lit off at a low
temperature and substrate temperature self-limited at about
750.degree. C. This catalyst cell density and gas flow rate
produced an intermediate gas temperature of 540.degree. C.
Similarly, stage 2 also self-limited the substrate temperature to
780.degree. C. and produced a gas temperature of 750.degree. C.
Stage 3 limited the wall temperature at 1100.degree. C.
Limiting the substrate temperature to 750.degree. C. to 780.degree.
C. for stages 1 and 2 provided excellent long term catalyst
stability. This stability was demonstrated for 100 hours as shown
in FIG. 5.
This catalyst system was again ignited by holding the inlet air
temperature at 400.degree. C. and increasing the fuel/air ratio by
increasing the methane flow rate. This start-up procedure is shown
in FIG. 6. Stage 1 achieved an outlet gas temperature of
540.degree. C. at fuel/air=0.033 and maintained this temperature at
fuel/air ratios up to 0.045. Complete homogeneous combustion after
the catalyst was achieved at a fuel/air ratio of 0.045.
This invention has been shown both by direct description and by
example. The examples are not intended to limit the invention as
later claimed in any way; they are only examples. Additionally, one
having ordinary skill in this art would be able to recognize
equivalent ways to practice the invention described in these
claims. Those equivalents are considered to be within the spirit of
the claimed invention.
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